The Sun pours down onto the Earth a vast quantity of radiant energy. Some of this energy, together with carbon dioxide and water, Nature traps as vegetation. A substantial part of the vegetation is a giant aromatic molecule that functions as a natural adhesive holding other parts of the plant together. This functioning has been copied by humans in the development of adhesives to glue together small pieces of wood to manufacture larger useful composite products exemplified by plywood, oriented strand board, particleboard and the like. The recognition of the chemical similarity between the synthetic and natural adhesives has repeatedly prompted the idea of using the natural adhesive in the man-made composite wood products in place of the synthetic. In spite of very extensive research efforts success has not been achieved for several distinct reasons. The first of these was economics. For many years the extremely low price of the main component of the synthetic adhesives, that was derived from petroleum, made its replacement by the natural adhesive marginal. With the rise in the value of petroleum this is no longer the case because the price of the derived main synthetic adhesive component has risen about fifteenfold. Of course, the natural aromatic adhesive has to be isolated from the vegetation and there are associated costs. In addition, the isolation procedures are usually part of reactions carried out on the vegetation for other reasons. The most common of these is the pulping of wood to yield fibers for papermaking. The aromatic natural adhesive polymer that is mostly available in abundance is designated kraft lignin and is part of the aqueous so-called “black liquor” as a polysodium salt. This liquor is usually concentrated and is burned in admixture with fuel oil to recover the inorganic chemicals therein. The fuel value of the liquor is very low. To isolate the kraft lignin from the black liquor requires acidification to precipitate the sodium-free form followed by filtration and drying of the gelatinous product. This is difficult to do and the isolated kraft lignin is therefore not inexpensive. Besides the cost factor of the processing, the isolated kraft lignin cannot be used as a wood adhesive component for two main technical reasons. The first is the wide range of molecular sizes present and the second, the relatively low level of reactive functionality with methyleneglycol, the other main component of the synthetic wood adhesive. The larger molecules in the kraft lignin molecular mixture cannot penetrate into the wood infrastructure so as to bond and reinforce the weakened surface layers damaged by the cutting or sawing processes that generated the small wood pieces.
The problem facing composite scientists has therefore been how to overcome these drawbacks to utilize polyphenolic materials from plants or plant-derived feedstocks in a large-scale, commercially practical, and energy efficient way such that the petroleum-derived components of wood adhesives can be at least partly replaced.
One of the most intriguing and environmentally sound approaches to breaking down molecules is simply to use water alone, heated to its supercritical state. About a decade ago this chemical-free technology was comprehensively discussed in an English language review by P. E. Savage (Chem. Rev. 1999, 99, 609). Since then few modern reviews have appeared. However, numerous articles, mostly from Japan and China, have appeared each year dealing with the reactive power of supercritical water. All of these publications emphasize that when water is heated to 374.4 C or above, the pressure concomitantly generated is 217.7 atm and above. The water then becomes a powerful new reactive solvent. Temperatures above 400 C seem to make the water even more effective in its new role. For example, it now dissolves nonpolar substances such as plant polyphenolics.
These and numerous other similar reactions (J. A. Onwudili & P. T. Williams, Chemosphere 2009, 74(6), 787) demonstrate clearly that chemical bonds can be broken down by treatment with supercritical water only, without the use of any catalysts. When a covalent single bond between two carbons atoms is cleaved, two free radicals are created, one on each carbon atom formerly at the ends of the single bond. These types of linkages join the aromatic rings making up much of the plant phenolic adhesive. The high reactivity of these free radical entities is probably involved in the formation of undesirable crosslinked and other undesirable large macromolecular complexes useless as adhesive precursors. These pathways are apparently blocked during the reactions of supercritical water which yields hydrogen atoms that combine with the free radical sites to deactivate them. This has actually been demonstrated by the use of deuterium oxide in place of water (hydrogen oxide) and the consequent finding of deuterium in the fragments. It has also been shown that sulfur-containing molecules can be desulfurized by cleavage of the carbon-sulfur bonds with expulsion of the sulphur atoms as hydrogen sulfide. Some of the established procedures for the isolation of the plant phenolic material introduce sulfur atoms into the isolated material. However, since nearly all water-substrate reactions have been run in a batch mode on a very small scale, the chemistry so elegantly elucidated there does not provide answers to the questions necessary for the future development of a commercially-sized, practical, continuous, supercritical water-based process.
The present invention fulfills these needs and provides for further related advantages.